1. Field Of The Invention
The present invention relates to imaging optical devices, and in particular to a wide field-of-view (WFOV) spectral filters and spectral sensors.
2. Background Of The Invention
Optical sensors have long been known and used for remote detection, identification and quantification of objects within a target scene based on the emissive spectra in the mid-wave infrared and long-wave infrared bands. Preferred for achieving a high level of operation of any type of sensors are high signal-to-noise ratio (SNR), fine spectral resolution, and wide area coverage. The two principal technologies available today are dispersive spectrometers and Fourier transform imaging spectrometers (FTIS).
Dispersive spectrometers have a long history in the short-wave infrared band. Dispersive spectrometers operate on what is referred to as a xe2x80x9cpushbroomxe2x80x9d principle. In such pushbroom devices, a 1xc3x97N pixel region of the image is scanned across the scene area, while the width of the image is determined by the field of view of the spectrometer and by the width of the focal plane array (FPA). A dispersive sensor passes the recollimated target beam from the slit off a grating, which diffracts the image to spread in a direction perpendicular to the width of the image, into a dispersive continuum collected by columns of pixels on the FPA. Each pixel along a collection column corresponds to a spectral bin, and the pixels in the row direction (cross scan direction) provide the spatial resolution.
The optical properties of such dispersive systems determine both the spatial and spectral resolutions of the sensor. Specifically, the spectral resolution is governed by the grating length and line pitch and, unlike the FTIS, cannot be varied. The resolution available in dispersive systems is much lower than in FTIS systems. However, the time per pixel to produce all the spectral information in a dispersive system is typically much shorter than in an FTIS. Therefore, the impact of wind, parallax and line of sight motion are all comparatively small for a dispersive system.
An FTIS passes light from a scene through two paths of different effective optical path lengths, and combines the light from the two paths on a FPA, conjugate to the scene. The optical path difference (OPD) between the two optical paths is produced by a traveling mirror, which inherently exhibits motion-associated biases and jitters. Spatial resolution is determined by the pixelized optical system, and spectral resolution is determined by the total stroke of the traveling mirror. As spectral resolution requirements increase, the stroke length increases. The minimum required SNR determines the length of time data is collected, which can be 1-20 seconds. During the time of the stroke, the optical system (which is mounted on a slewing platform such as an airplane) must maintain precise registration of the scene. Changes in the image due to parallax, magnification, aspect angle, vehicle motion, wind and changes in the optical system due to drift and jitter, all combine to misregister the interferogram data cube. The result is degraded spectral resolution, spatial resolution and spectral noise.
Therefore, each of the two technologies has significant drawbacks that affect the overall quality of their performance. Accordingly, there is a need to provide an improved hyperspectral WFOV sensor providing the spectral resolution of a FTIS with the short integration times and fast area collection of a pushbroom dispersive system, while still being relatively simple and inexpensive to manufacture.
The present invention is directed to WFOV spectral filters and spectral sensors.
Optical filters with sinusoidal output versus wavenumber incorporated in spectral sensors can greatly improve system speed and other performance parameters. The sinusoidal performance is achieved by developing a specific OPD between two components of the light from a given scene. The OPD can be achieved with either a Michelson, Mach-Zender or birefringent interferometer. Utility of an interferometer is greatly enhanced if its OPD is nearly constant over a WFOV. Thus, in a first, separate aspect of the present invention, a WFOV in a filter is achieved by choosing elements and dimensions inside the filter such that the following requirement is satisfied: the first and second derivatives of the optical path difference inside the filter over the incidence angle of the incoming beam equals zero. From a mathematical point of view, the requirements             ⅆ              ⅆ        θ              ⁢    OPD    =            0      ⁢              xe2x80x83            ⁢      and      ⁢              xe2x80x83            ⁢                        ⅆ          2                          ⅆ                      θ            2                              ⁢      OPD        =    0  
means no significant dependence of the OPD over the incidence angle xcex8. From a practical point of view, this formula allows the calculation of necessary dimensions inside a filter.
In a second, separate aspect of the present invention, field of view capability is enhanced in an interferometer of the Michelson or Mach-Zender types by using gap plates of specific refractive indices and thicknesses inside such an interferometer. The calculations based on the idea that             ⅆ              ⅆ        θ              ⁢    OPD    =            0      ⁢              xe2x80x83            ⁢      and      ⁢              xe2x80x83            ⁢                        ⅆ          2                          ⅆ                      θ            2                              ⁢      OPD        =    0  
provide the mathematical interrelations of a given OPD, the thicknesses of the plates, and their refractive indices.
In a third, separate aspect of the present invention, the field-of-view capability of an interferometer is enhanced in a situation where the target scene is illuminated by narrow band radiation such as LED (or, in the alternative, where a sensor receives information from a narrow band filter located between the target scene and the sensor). In such a situation, a WFOV is received in the plane of a narrow slit. Information from the scene passes through the narrow slit into the sensor, while maintaining a modest field-of-view in other planes. In the case of a Mach-Zender type interferometer, this is achieved by using a plate in place of both beam-splitters and gap plates. The plate of refractive index n should have a thickness       t    p    =      OPD    ·                  cos        ⁢                  xe2x80x83                ⁢                  φ          i                            2        ·                  (                      n            -                          1              n                                )                    
that, after appropriate calculations, readily follows from the venerable equation                     ⅆ        2                    ⅆ                  θ          2                      ⁢    OPD    =  0.
In a fourth, separate aspect of the present invention, a one-dimensional wide field of view can be created with a uniaxial birefringent interferometer. Birefringent crystals propagate two linear polarizations with distinct phase velocities. If such a crystal is used to propagate two components of an incoming beam with different polarizations, then locating the scene slit at an angle xcex8 with respect to the ne crystal axis will result in a WFOV along the slit, wherein             tan      ⁢              xe2x80x83            ⁢              θ        2              =                  n        e                    n        0              ,
and no and ne are the crystal""s ordinary and extraordinary indices of refraction, respectively.
In a fifth, separate aspect of the present invention, gap plates creating a WFOV are used in a cascade of chained Mach-Zender interferometers. The combination of interferometers with enhanced field-of-view capabilities result in, a separation of optical signals having different wavenumbers and improved spectral resolution uniformly over the WFOV. Because, by design, the OPD of each stage within the system is twice that of the succeeding stage, the determination of the dimensions of the gap plates within one interferometer will dictate the dimensions of the plates within all other interferometers of the system.
In a sixth, separate aspect of the present invention, a cascade of chained interferometers, or hyperspectral filters, is used in combination with a dispersive spectrometer to create a high-speed high-resolution hyperspectral sensor. Such a sensor combines the high SNR and large area collection capacity of a dispersive spectrometer with the excellent spectral resolution of an FTIS. An integration of a cascade of hyperspectral filters with a dispersive system consists of inserting the interferometers between the grating and the imaging lens of a dispersive system, and replicating the lens and focal plane array for each of the output channels of the interferometers.
Accordingly, it is a primary object of the present invention to provide an optical sensor with enhanced field-of-view and area collection capabilities which achieves fine spectral resolution while retaining a short measurement time per pixel. This and further objects and advantages will be apparent to those skilled in the art in connection with the drawings and the detailed description of the preferred embodiments set forth below.